Abstract : Coadsorption of water with either ammonia or hydrogen fluoride on a Ag(110) substrate was studied to examine the relationships between intermolecular hydrogen bonding near the surface and preferential adsorption at the surface. The experiments were conducted in ultrahigh vacuum with facilities for thermal desorption spectroscopy and high resolution electron energy loss spectroscopy. Adsorption was performed at 110 K for ammonia and water and at 90 K for hydrogen fluoride and water. Ammonia and water simply coadsorb at 110 K with no evidence of specific hydrated complexes nor of ionization to NH4(+). Coadsorbed water enhances population of the chemisorbed state of ammonia, which we estimate to have a saturation coverage of 0.12 monolayer without water, through a mechanism of dielectric screening. At coverages approaching one monolayer and higher, water stabilizes the bulk of ammonia, increasing the average desorption temperature by as much as 21 K from 134 K. jg p.4